1. Field of the Invention
This invention relates to position encoders and more particularly to position encoders which utilize capacitive coupling in order to provide a position indication. Still more particularly, the present invention is directed to a capacitive encoder for use in positioning the read/write heads in a rotating memory system.
Rotating memories, i.e., disc type memories, store data in a circular fashion. The data in the memory is accessed by moving a magnetic head across the radius of the disc. The trend in the design of rotating memories is to increase the memory capacity through higher track density and higher bit density. Recent designs in hard disc ("Winchester") systems have achieved track densities on the order of 700 tracks per inch. Typically, the magnetic head used to read data is positioned by means of a servomotor which drives an arm to which the head is attached.
The trend toward high track density has created the need for a high resolution positioning system to accurately locate the head with respect to the tracks. This is accomplished by employing an encoder to determine the position of the shaft of the servomotor to thereby determine the head position. The encoder must be able to provide precise velocity and direction information and should have low inertia and high stability.
2. Description of the Prior Art
Most encoders presently in use in rotating memory systems are optical encoders which generally include a metal or glass disc having a series of apertures or markings, a mask and an optical system. The output of such encoders cannot be described precisely in mathematical form and digitizing of the output is imprecise. Furthermore, this type of encoder often has significant stability problems. In addition, such encoders are generally expensive, expecially if high resolution is required.
Although not generally employed with rotating memory head positioners, various other position encoders have been developed which employ capacitive coupling between a pair of moving plates. Encoders of this type are disclosed in U.S. Pat. Nos. 3,961,318 to Farrand, et al., 3,219,920 to Wall, 3,938,113 to Dobson, et al., 3,702,467 to Melnyk, 3,784,897 to Norrie, 3,312,892 to Parnes, 3,068,457 to Nevius, and 4,040,041 to Fletcher, et al. These encoders typically include a first plate having a pair of conductive input areas which are separated by a gap having a periodic waveform and a second plate in facing relation to the first plate which includes conductive output areas which capacitively couple to the input areas by an amount which is determined by the relative position of the two plates.